ath79: add support for Comfast CF-WR752AC v1

ath79: add support for Comfast CF-WR752AC v1

Specifications:

- Qualcomm QCA9531 + QCA9886
- dual band, antenna 2*3dBi
- Output power 50mW (17dBm)
- 1x 10/100 Mbps LAN RJ45
- 128 MB RAM / 16 MB FLASH (w25q128)
- 3 LEDs (red/green/blue)
  incorporated in
  "color wheel reset switch"
- UART 115200 8N1

Flashing instructions:

 The U-boot bootloader contains a recovery HTTP server
 to upload the  firmware. Push the reset button while powering the
 device on and keep it pressed for ~10 seconds. The device's LEDs will
 blink several times and the recovery page will be at
 http://192.168.1.1; use it to upload the sysupgrade image.

 Alternatively, the original firmware is based on OpenWrt so a
 sysupgrade image can be installed via the stock web GUI. Settings from
 the original firmware will be saved and restored on the new one, so a
 factory reset will be needed. To do so, once the new firmware is flashed,
 enter into failsafe mode by pressing the reset button several times during
 the boot process, until it starts flashing. Once in failsafe mode, perform
 a factory reset as usual.

LED-Info:

 The LEDs on the Comfast stock fw have a very proprietary behaviour,
 corresponding to the user selected working mode (AP, ROUTER or REPEATER).
 In the first two cases, only blue is used for status and LAN signaling. When
 using the latter, blue is always off (except for sysupgrade), either red
 signals bad rssi on master-link, or green good. Since the default working
 mode of OpenWrt resembles that of a router/AP, the default behavior is
 implemented accordingly.

MAC addresses (art partition):

location  address (example)    use in vendor firmware
0x0       xx:xx:xx:xx:xc:f8 -> eth0
0x6       xx:xx:xx:xx:xc:fa -> wlan5g (+2)
0x1002    xx:xx:xx:xx:xc:f9 -> not used
0x5006    xx:xx:xx:xx:xc:fb -> not used
---       xx:xx:xx:xx:xd:02 -> wlan2g (+10)

The same strange situation has already been observed and documented
for COMFAST CF-E560AC.

Signed-off-by: Roman Hampel <rhamp@arcor.de>
Co-developed-by: Joao Albuquerque <joaohccalbu@gmail.com>
Signed-off-by: Joao Albuquerque <joaohccalbu@gmail.com>
[adjust and extend commit message, rebase, minor DTS adjustments,
add correct MAC address for wmac, change RSSI LED names and behavior]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>